Molecular sieves having a wide variety of compositions and structures have been disclosed in the art as useful in catalysts for hydrocarbon conversion. The most well known are the crystalline aluminosilicate zeolites formed from corner-sharing AlO.sub.2 and SiO.sub.2 tetrahedra. The zeolites generally feature pore openings of uniform dimensions, significant ion-exchange capacity and the capability of reversibly adsorbed an adsorbed phase which is dispersed throughout the internal voids of the crystal without displacing any atoms which make up the permanent crystal structure. Zeolites often are characterized by a critical, usually minimum, silica/alumina ratio.
More recently, a class of useful non-zeolitic molecular sieves containing framework tetrahedral units (TO.sub.2) of aluminum (AlO.sub.2), phosphorus (PO.sub.2) and at least one additional element EL (ELO.sub.2) has been disclosed. "Non-zeolitic molecular sieves" include the "ELAPSO" molecular sieves as disclosed in U.S. Pat. No. 4,793,984 (Lok et al.) and "SAPO" molecular sieves of U.S. Pat. No. 4,440,871 (Lok et al.). Generally the above patents teach a wide range of framework metal concentrations, e.g., the mole fraction of silicon in Lok et al. '871 may be between 0.01 and 0.98 depending on other framework elements with a preferable range of 0.02 to 0.25 mole fraction. U.S. Pat. No. 4,943,424 (Miller) discloses a silicoaluminophosphate molecular sieve characterized by surface and bulk P.sub.2 O.sub.5 -to-alumina ratios in the surface and bulk of the sieve and silicon content of the surface and its use in dewaxing and hydrocracking.
U.S. Pat. No. 4,740,650 (Pellet et al.) teaches xylene isomerization using a catalyst containing at least one non-zeolitic molecular sieve which preferably is a silicoaluminophosphate. Pellet et al do not suggest the critical composition gradients which are a feature of the present invention.
Catalysts for isomerization of C.sub.8 aromatics ordinarily are classified by the manner of processing ethylbenzene associated with the xylene isomers. Ethylbenzene is not easily isomerized to xylenes, but it normally is converted in the isomerization unit because separation from the xylenes by superfractionation or adsorption is very expensive. A widely used approach is to dealkylate ethylbenzene to form principally benzene while isomerizing xylenes to a near-equilibrium mixture. An alternative approach is to react the ethylbenzene to form a xylene mixture via conversion to and reconversion from naphthenes in the presence of a solid acid catalyst with a hydrogenation-dehydrogenation function. The former approach commonly results in higher ethylbenzene conversion, thus lowering the quantity of recycle to the para-xylene recovery unit and concomitant processing costs, but the latter approach enhances xylene yield by forming xylenes from ethylbenzene. A catalyst composite and process which enhance conversion according to the latter approach, i.e., achieve ethylbenzene isomerization to xylenes with high conversion, would effect significant improvements in xylene-production economics.